Resistance variable memory elements, which include Programmable Conductive Random Access Memory (PCRAM) elements, have been investigated for suitability as semi-volatile and non-volatile random access memory elements. An exemplary PCRAM device is disclosed in U.S. Pat. No. 6,348,365, which is assigned to Micron Technology, Inc.
A PCRAM device typically includes chalcogenide glass as the active switching material. A conductive material, such as silver, is incorporated into the chalcogenide glass creating a conducting channel. During operation of the device, the conducting channel can receive and expel metal ions (e.g., silver ions) to program a particular resistance state (e.g., a higher or a lower resistance state) for the memory element through subsequent programming voltages, such as write and erase voltages. After a programming voltage is removed, the programmed resistance states can remain intact for a period of time, generally ranging from hours to weeks. In this way, the typical chalcogenide glass-based PCRAM device functions as a variable resistance memory having at least two resistance states, which define two respective logic states.
One exemplary PCRAM device uses a germanium selenide (i.e., GexSe100−x) chalcogenide glass as a backbone along with silver (Ag) and silver selenide (Ag2+/−xSe). See for example U.S. Patent Application Publication No. 2004/0038432, assigned to Micron Technology, Inc.
Although the silver-chalcogenide materials are suitable for assisting in the formation of a conductive channel through the chalcogenide glass layer for silver ions to move into, other non-silver-based chalcogenide materials may be desirable because of certain disadvantages associated with silver use. For example, use of silver-containing compounds/alloys such as Ag2Se may lead to agglomeration problems in the PCRAM device layering and Ag-chalcogenide-based devices cannot withstand higher processing temperatures, e.g., approaching 260° C. and higher. Tin (Sn) has a reduced thermal mobility in GexSe100−x compared to silver and the tin-chalcogenides are less toxic than the silver-chalcogenides.
Research has been conducted into the use of thin films of SnSe (tin selenide) as switching devices under the application of a voltage potential across the film. It has been found that a 580 Å SnSe film shows non-volatile switching between a higher resistance state (measurable in MOhm) and a lower resistance state (measurable in kOhm) when potentials of 5-15 V are applied by forming an Sn-rich material (e.g., a dendrite). Also, the addition of Sn to a GexSe100−x glass, which is a chalcogenide glass, has been found to produce memory switching if a high enough potential, e.g., >40 V, is applied across the chalcogenide glass. However, such switching potentials are too high for a viable memory device.
One time programmable (OTP) memory cells are known and have many applications. A typical OTP memory cell may function as a fuse or an antifuse. In a memory device application, such a fuse or antifuse may be connected between a column line and a row line. In a memory cell having a fuse, a charge sent through the column line will pass through the intact fuse in a cell to a grounded row line indicating a value of 1. To change the value of a cell to 0, a specific amount of current is applied to the cell to burn out the fuse. In a cell having an antifuse, the initial unprogrammed state a 0, and the cell is programmed to a 1 state. Once the conventional OTP cells are programmed, they cannot be erased or reprogrammed.
Accordingly, it is desired to have a resistance variable memory element which can act as an OTP or OTP-like memory cell. Additionally it is desirable to have such a memory element that can be reprogrammed at least once after an initial programming.